Braking saccade--a new fast eye movement.

A new type of fast eye movement (FEM) is described whose main function is to stop a runaway slow eye movement (SEM). It conforms to the velocity-amplitude relationship characteristic of other types of saccades. The identification of this FEM is the result of examination of the many waveforms manifested by subjects with congenital nystagmus. It is a common, repeatable phenomenon present in all subjects with any of six different types of nystagmus waveform. The fact that braking saccades reset SEM to zero velocity, whereas no other type of saccade interacts with SEM, indicates a developmental mechanism in such subjects.